Vehicular tires may be prepared with a component of a rubber composition which contains reinforcing filler comprised of a combination of precipitated silica and rubber reinforcing carbon black. The precipitated silica is usually provided with a silica coupling agent (silica coupler) to enhance its rubber reinforcement by coupling precipitated silica to a conjugated diene based elastomer contained in the rubber composition.
Precipitated silica is generally hydrophilic in nature and therefore has a greater affinity to itself, namely to other hydrophilic precipitated silicas, with less affinity to a diene-based elastomer in a rubber composition which, in turn, makes it difficult to obtain a satisfactory dispersion of the precipitated silica throughout the rubber composition. Such difficulty of obtaining a dispersion of hydrophilic precipitated silica is well known to those having skill in such art.
In practice, the precipitated silica may be hydrophobated to promote a greater affinity to a diene based elastomer in a rubber composition and to thereby enable a more satisfactory dispersion of precipitated silica in a rubber composition.
The precipitated silica may be hydrophobated, for example, with an alkylsilane in a form of, for example, a halogenated alkylsilane or alkoxyalkylsilane to react with hydroxyl groups on the precipitated silica. It may also be hydrophobated with a silica coupling agent where the coupling agent contains a moiety (e.g. alkoxysilane moiety) to react with hydroxyl groups on the silica and another different moiety (e.g. polysulfide moiety) interactive with the diene-based elastomer(s) of the rubber composition to thereby render the precipitated silica more readily dispersible in the rubber composition.
Precipitated silica may be pre-hydrophobated prior to its introduction into the rubber composition or may be in situ hydrophobated within the rubber composition.
However, it is readily seen that such pre-hydrophobation of the precipitated silica with an alkylsilane (by way of a reaction with a halogenated alkyl silane or alkoxysilane) can render the hydrophobated precipitated silica significantly unreactive, or only minimally reactive, with a subsequent addition of silica coupling agent. Such pre-hydrophobation thereby eliminates a significant portion of hydroxyl group sites on the precipitated silica available to subsequently react with a silica coupling agent which is added subsequently in situ within the rubber composition. Therefore, while such pre-hydrophobation promotes dispersion of the precipitated silica in the rubber composition, it can prevent significant subsequent coupling of the precipitated silica to a diene-based elastomer contained in a rubber composition with a silica coupling agent.
Exemplary of hydrophobation of precipitated silica, and not intended to be limiting, may be found, for example, U.S. Pat. Nos. 4,474,908, 5,780,538 and 6,573,324.
For this invention, it is desired to evaluate dispersion (dispersability) of precipitated silica in a rubber composition containing a diene-based elastomer followed by coupling of the dispersed precipitated silica to a diene based elastomer contained in the rubber composition to thereby both promote stabilization of the dispersed precipitated silica within the rubber composition and to promote reinforcement of the rubber composition.
To enable such combination, it is desired to evaluate partially pre-hydrophobating a precipitated silica to promote its dispersability in the rubber composition while promoting a significant retention of residual unreacted hydroxyl groups on the precipitated silica followed by coupling the dispersed partially pre-hydrophobated precipitated silica by reaction of its residual hydroxyl groups with a silane containing (e.g. alkoxysilane containing) silica coupling agent in situ within a rubber composition which contains a conjugated diene based elastomer.
It therefore is desired to provide such partial pre-hydrophobation of precipitated silica with the alkylsilane exclusive of silica coupling agent during such pre-hydrophobation to prevent an inherent competition of the alkylsilane with silica coupling agent for hydroxyl groups on the precipitated silica during such partial pre-hydrophobation.
While such partial pre-hydrophobation of precipitated silica to promote its dispersibility combined with a subsequent and separate coupling with a coupling agent to a rubber composition may appear to be relatively straight forward in nature, it is however believed an innovative methodology and resultant product of this application is a significant departure from past practice. (for example, see U.S. Pat. Nos. 6,573,324, 6,830,811 and 7,569,107), where hydrophobation of a precipitated silica was provided with a combination of alkylsilane and silica coupling agent.
The partial pre-hydrophobation of the silica surface with an alkylsilane prior to the in-situ addition of the coupling agent allows for simple dispersion of the partially pre-hydrophobated silica within the rubber matrix prior to coupling of that pre-hydrophobated silica with the rubber matrix with the silica coupler through residual hydroxyl groups remaining on the precipitated silica after its pre-hydrophobation. The subsequent reaction of a silica coupler is provided to stabilize the dispersed pre-hydrophobated silica within the rubber as well as to reinforce the rubber itself by reaction of the added silica coupler with available and accessible residual hydroxyl (e.g. silanol) groups contained on the pre-hydrophobated silica, and which may also be contained on the pre-hydrophobating alkylsilica compound and with the diene based elastomer of the rubber composition.
For this evaluation, it is desired to partially pre-hydrophobate a precipitated silica by reaction of an alkylsilane, in a form of a halogenated alkylsilane or an alkoxyalkylsilane compound. By such partial pre-hydrophobation, it is desired that a significant content of residual unreacted hydroxyl groups on the precipitated silica remains available for subsequent reaction with a silica coupling agent in situ within a rubber composition and to thereby couple the precipitated silica to a conjugated diene elastomer contained in a rubber composition.
By such methodology, it is envisioned that dispersion of the precipitated silica in the rubber composition is promoted, followed by stabilization of the dispersed precipitated silica in the rubber composition and reinforcement of the rubber composition by the dispersed pre-hydrophobated precipitated silica is provided.
It therefore is desired to provide such partial pre-hydrophobation of precipitated silica with the alkylsilane exclusive of silica coupling agent during such pre-hydrophobation to prevent an inherent competition of the alkylsilane with silica coupling agent for hydroxyl groups on the precipitated silica during such pre-hydrophobation.
In the description of this invention, the term “phr” relates to parts by weight for a material or ingredient per 100 parts by weight elastomer(s)”. The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.